Glass used in buildings and vehicles, generally protects us from the environment (rain, wind, noise, etc.), allowing more pleasant conditions inside. However, ordinary glass does not protect us from solar radiation, since it only absorbs some of the UV radiation, reflecting a total of about 7% and transmits most of the entire solar spectrum. In particular, in the case of automobiles, the trend is to use larger area and more inclined (relative to the horizontal) (front) windshields, thus substantially increasing the amount of incoming solar radiation, reaching about 35% of total heat entering the vehicle, which corresponds to ˜50% of heat input only through the windshield. This requires improvements in the properties of glass (by coating) to reduce infrared input improving passenger comfort, increasing the service life of the vehicle interior furnishings (console, carpets, etc.) and reducing the use of air conditioning thereby saving fuel; this is what is known as solar control.
Value added to glass or other products may be increased depending on the functional properties conferred to its surface or some coating deposited on it. Many phenomena that give functional characteristics to a material occur on the surface or in a region close to it. It is therefore possible to coat economical substrates (glass) with functional materials in the form of thin layers. Thus, the resulting product has the functional property of the coating and the characteristics of the substrate, particularly those of glass.
Solar control refers to the ability to change the amount of transmitted or reflected radiation, in the near-UV (UV; 300-380 nm), visible (VIS; 380-780 nm) and infrared (IR; 780-2500 nm) spectral ranges. Low transmittance is generally pursued in UV and IR ranges, while the VIS transmittance may be high (>70%) or low, depending on the application.
In addition to blocking infrared radiation, glass and its coatings must have other properties, such as: high transmittance in the visible spectrum (>70%), high mechanical strength, chemical resistance and weather resistance, they must be able to undergo thermal treatments (tempering, bending), must show a neutral color without iridescence, low dispersion (haze) and be low cost. The aggregate of necessary properties makes the development of this type of coatings a technically complex and very difficult problem.
There are many alternatives to obtain solar control properties. This is reflected in the myriad of scientific papers, patents and patent applications existing on the subject. For example, one scientific publication referring to coatings with solar control properties, is the paper “Solar heat reflective glass by sol-gel nanostructured multilayer coatings” by Z. Nagamedianova and colleagues, published in the journal Optical Materials in 2011, Volume No. 33, pages 1999-2005 describing commercial clear glass coated by the sol-gel method with three layers of oxides, TiO2—SiO2—TiO2, which have the property of reflecting the IRC (near-IR). Transmittance in the VIS >70%, high UV blocking (Tuv <35%) and high reflectivity (>60%) in the 800 to 950 nm range are reported.
Regarding patents, U.S. Pat. No. 5,242,560 “Heat treatable sputter-coated glass” by Guardian Industries Corp. describes a coated glass that may be heat treated by sputtering, consisting of a layer of Ni alloy with one or two layers of Sn oxide, and optionally an intermediate Al layer.
The published US Patent Application No. 2011/0236715 A1 relates to a “Solar control coating with discontinuous metal coating layer” owned by PPG Industries Ohio, Inc. Said application proposes a coating deposited on at least a portion of a substrate, comprising multiple dielectric layers alternating with multiple metal layers, with at least one of the metal layers comprising a discontinuous metal layer.
In British Patent (1971) No. 1241889 “Heat reflecting glass and method for manufacturing the same” owned by Asahi Glass Co., a glass substrate which reflects heat and transmits visible light, comprised by a composite of a metal oxide layer (TiO2, Ta2O5, WO3, ZrO2, Nb2O5, ThO2, SnO2) of higher index than glass, in which microscopic particles of metallic Pd or Au are immersed, is claimed. The proposed method is similar to Sol-gel.
Furthermore, there are several methods of synthesis of coatings including: sol-gel, pulsed laser deposition, vacuum evaporation, electron beam, sputtering, CVD and plasma discharge, which includes the variant called AACVD. Among these preparation techniques, the AACVD method has some advantages such as: its simplicity and low cost of implementation, since it needs no sophisticated equipment, ability to operate at atmospheric pressure and it is scalable to industrial level. This technique allows obtaining coatings with several advantages: a) controllable composition, even when changing the composition of a precursor solution during deposition with the purpose of obtaining materials with a concentration gradient, b) good adhesion, c) uniform and controllable thickness over a wide range, d) ease of production of composite materials or multiple layers e) it can be applied for depositing coatings on flat substrates or on inner or outer pipe surfaces, f) finally the properties of the materials obtained are comparable to those of materials deposited by other more sophisticated techniques such as reactive sputtering, reactive evaporation, PLD, etc. which require expensive high vacuum systems, radio frequency sources, gas control, power laser, etc.
The AACVD method is a physical chemical hybrid process for obtaining coatings. It consists in producing a cloud of micrometric drops, from a solution composed of organometallic precursors or inorganic compounds, dissolved in a particular solvent for each type of compound (water, alcohol, acetone, acetylacetone, etc.). The aerosol can be generated by pneumatic, electrostatic or ultrasonic methods. The aerosol precursor solution must be transported to the deposition area by a carrier gas. In the deposition area is the glass substrate, which is heated to a specific temperature depending on the material to be deposited or the precursors used. In the deposition area, as the cloud approaches the substrate it warms up causing initially solvent evaporation, fusion, evaporation or possible sublimation or thermal decomposition of the precursor compound, its diffusion towards the glass surface; where the process continues with the adsorption of the reactants, the chemical reaction, and its evacuation away from the surface.
Some scientific publications referring to systems for production of thin coatings by the AACVD method are:
The paper “Aerosol-Assisted Chemical Vapor Deposited Thin Films for Space Photovoltaics” by Aloysius F. Hepp et al, published by National Aeronautics and Space Administration NASA/TM-2006-214445 describing different reactor designs at atmospheric pressure and low pressure, analyzing their main parameters determining the deposition of thin semiconductor coatings based on In and Cu sulfides for photovoltaic applications. The area of application of these coatings differs from those proposed in the present invention.
Another report “Synthesis, structural characterization and optical properties of multilayered Yttria-stabilized ZrO2 thin films obtained by aerosol assisted chemical vapour deposition” by P. Arnézaga-Madrid, W. Antúnez-Flores, L Monárrez-García, J. González-Hernández, R. Martínez-Sánchez, M. Miki-Yoshida, published in the journal Thin Solid Films in 2008 with number 516, pages 8282-8288, describes how to obtain multilayer coatings of yttria-stabilized zirconia on borosilicate glass substrates by the AACVD method. The paper discusses the influence of various synthesis conditions such as: concentration of the precursor solution, substrate temperature, carrier gas flow, etc., on the coating growth rate. The multilayer structure obtained due to the iterative process used allows modulating the refractive index, thus modifying the reflection of the coating.
There are also patents which relate to CVD (chemical vapor deposition) systems for production of thin films on flat substrates, for example, U.S. Pat. No. 6,190,457 B1 describes a horizontal CVD system for obtaining a thin film semiconductor compound made up of two or more components on the surface of a flat substrate. The CVD system has a cylindrical reactor and a flat substrate is placed inside the reactor. The reactor has a gas supply section and a section for its disposal. Inside the reactor there are three divisions; in the first two divisions a gas mixture made up by one that includes the precursor compounds and another diluent gas. In the third section only an inert gas is fed which transports the two previous mixtures.
The U.S. Pat. No. 7,011,711 B2 presents a vertical system using the method of chemical vapor deposition for producing a thin film on one or more flat substrates. The system comprises a reactor including a vertical pipe and a reaction chamber located inside the pipe. The flat substrate is placed at the end of the reaction chamber. Gas input and exhaust is carried out vertically. Throughout the length of the pipe, partition arrangements are positioned to direct the path of the reaction gases and to evacuate the gases produced after the reaction. Additionally, heaters are connected to the vertical pipe that can control the temperature difference between the substrate and the reactor walls.
Considering the aforementioned technique, the present invention relates to a coating with solar control properties deposited on glass intended for architectural or automotive use, either monolithic or laminated. The coating consists of several layers of different metal oxide semiconductors (TiO2, ZnO, ZrO2, Al2O3) with different refractive index (n), and a layer of metal nanoparticles (Au, Ag). The layer of metal nanoparticles increases IR blocking. Additionally, the use of n type metal-semiconductor active junctions, above and below the nanoparticle layer, allows the injection of negative charges from the metal to the semiconductor (Schottky junction) protecting it from oxidation and also preventing metal agglomeration, to obtain nanoparticles homogeneously deposited throughout the coating. The coating component layers are superimposed in a predetermined order, such as: glass (VC)/diffusive barrier (BD)/dielectrics 1 (DI)/n-type semiconductor, adhesive-protector (A)/metal nanoparticles (M)/n-type semiconductor, protector (P)/dielectrics 2 (D2)/mechanical strength (R); thicknesses are selected so that the coating confers to glass solar control properties, especially a high near-IR blocking (CRI) and high transmittance in the VIS. The number of coating layers may vary being at least three, composed of two n-type semiconductors, distributed below and above the layer of metal nanoparticles.
The coating was obtained by using the aerosol-assisted chemical vapor deposition technique. This technique uses precursor solutions consisting of a salt containing the element to be deposited, for example titanyl acetylacetonate or aluminum acetylacetonate, along with a suitable solvent such as methanol, ethanol, water or some other solvent for completely dissolving the precursor salt. A pneumatic, ultrasonic or electrostatic type nebulizer converts the precursor solution to a cloud of micrometric drops, which are driven by a carrier gas, usually air, toward the glass surface that is at deposition temperature between 100 and 600° C. The particular temperature required depends on the material to be deposited, in other words, on the precursor used. The process is repeated successively with the different precursors to deposit all the layers of the coating.